[Tomotada / Kiwamu]
The open loop transfer function of the Y end PDH loop was remeasured : the UGF was found to be at 17 kHz.
The phase margin at the UGF was about 27 deg.
While the measurement we noticed that the modulation onto the laser PZT was too big
and it was creating a big AM on the reflected light with an amplitude of a few mV.
So we put a 20 dB attenuator to decrease the modulations and the reflected light became much quitter.
Also the servo shape formed by Newfocus LB1005 looks too simple : we should have a more sophisticated servo filter (i.e. PDH box!!).